Imagine: remove bogus assets

- flat buttons don't need checkable assets (checkable-checked vs.
  checked, and checkable-highlighted vs. highlighted look identical)
- delegates don't need a checked background, because the checked
  state is visualized by the indicator
- text editors have no pressed state
- mirrored page-indicator is unnecessary because it looks identical
  (if a mirrored version was needed, then we would need to provide
   mirrored versions of all states including pressed, current...)

Change-Id: I909f639bdcf9d9653c56b8ce2eda49b3219bdf77
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>